The present invention relates to an optical data storage medium and to methods for recording and reading data on/from such medium.
More specifically, a concept is described for optical storage of information on data carriers such as fiches, cards or tape.
The data carrier's surface is completely or partly covered with focusing microstructures adjacent to a layer which is able to change its optical properties when exposed to intense light. (In the following, this layer will for simplicity in most cases be referred to as the "burn-film". This expression does not imply any particular embodiment of the layer, which can be of the reversible or the irreversible type, as discussed below.) During recording of data, local changes are created in the layer by having the microstructures focusing the light, thus obtaining a high light intensity on the layer. During reading of data, the optical microstructures may operate as an active optical component assisting the reading equipment.
By using e.g. transparent microspheres (typical diameter: 1 to 100 .mu.m) as focusing elements disposed over an optically absorbing film, a tight focusing and a high data storage capacity can be achieved without the complexity and the expenses that otherwise would be required for a recording and reading system which focuses directly on the light absorbing layer. Using oblique illumination through the microspheres, possibilities arise for storage of a high number of data bits at each microsphere position, as well as for hierarchically built data bases, in certain cases combined with data protection. Easy prerecording on mass produced data carriers becomes possible. During reading of data the microspheres function as auxiliary optics, which makes it possible e.g. to read large blocks of data without the use of a laser.
Storage of data by means of laser beams which produce local change of the optical properties of a thin film on a planar substrate are well known, for example from SPIE vol. 329 (1982), SPIE vol. 490 (1984) , SPIE vol. 695 (1986), SPIE vol. 899 (1988), SPIE vol. 1078 (1989). The change in optical properties can be reversible, whereby the stored data can be deleted and replaced by new data.
Alternatively, the change in optical properties can be irreversible, whereby it becomes impossible to delete and/or rerecord new data. Storage media of the latter type are often referred to as WORM (Write Once Read Many Times) media.
A usual method of preparation of WORM media consists of depositing on the substrate, which is a plastic disc, a thin film of a low melting point metal such as Te. During data storage each bit is represented by the physical status of the specific film area assigned for the storage of said bit (one elementary data storage cell with its address), i.e. whether that area has been irreversibly changed due to exposure to light or whether it is unchanged. According to present knowledge, all practical optical data storage systems are so far based on reflection from the burn-film. The irreversible change consists in such a case of an increase or a decrease of the reflectivity of each cell when a focused laser beam heats the burn-film. In the film is thereby created a hole through which the light can pass, or the film is smoothened so that its reflectivity increases. Several other processes can be used, e.g. local deformation of the substrate which influences the reflectivity. Reading is most commonly performed by examining the reflectivity of each cell by means of a focused laser beam scanning the surface of the data carrier systematically. This laser beam is too weak to influence the reflectivity.
From NO patent application 86.4041 (=DE 35 36 739)a data storage medium of the optical type is known, which medium is equipped with focusing optical structures integrated in the medium together with and on top of a material the optical properties of which can be changed by irradiation with light. This known data carrier is however only adapted for storage of visible data, i.e. images which are visually perceptible and which consist of spots that are to be interpreted together in order to create an image. "Tilt-images" can be created due to the optical structures, i.e. a lens raster or array, because the lenses are able to direct light to defined small areas underneath the lenses, depending on the direction of illumination. The lenses are however, relatively large, the range of variation of the lens diameter is indicated to be 150-500 .mu.m, recommended about 400 .mu.m. Thus, the technique described does not relate to a data storage medium for optimized "close packing" of e.g. digital, independently interpretable data bits, but only a spot-structured image storage of the directly visible type. Only irreversible recording of data is described in NO patent application 86.4041.
A main objective for the use of optical data storage media instead of magnetic ones is the very high storage density which can be achieved, combined with good long-time stability (e.g. immunity against magnetic fields). Optical data storage is however subject to both fundamental and practical limitations, and a trade-off must be made between different desired features, of which the most important are:
1) Large number of stored bits per unit area PA1 2) Low laser energy for recording of each bit PA1 3) High contrast when reading information PA1 4) Fast recording and reading PA1 5) Short "random access" time PA1 6) Robust and stable data storage medium PA1 7) Inexpensive data storage medium PA1 8) Inexpensive recording and reading equipment PA1 9) Moderate requirements for precise focusing (distance control)
Today's techniques have various limitations/problems. Consider first the recording and reading systems: If a high data storage density is desired it is necessary to use for data recording a laser beam which is focused on the burn-film by means of very high speed optics (lens with low f-number). Theoretically, the light beam can under optimum conditions be focused to a diameter of about the wave length of the light which is applied. Practical systems have been realized which come very close to that, with focal point diameters of 0.5-1 .mu.m. However, under such strong focusing the requirements for the mechanical control of the lens become very strict.
Lens to burn-film distance must be controlled precisely: On either side of the focal point the laser beam diverges very quickly and in a distance of z/2 from the focus the diameter is doubled (provided a Gaussian light beam, see FIG. 1). z can be interpreted as a measure of "depth of field" which is correlated to the beam diameter w at the focal point and the light wave length .lambda. by the equation: EQU z=.pi.w.sup.2 /.lambda. (1)
Since w is very small in this connection, z lies only in the range 1-10 .mu.m, which means that the distance between lens and storage medium must be controlled with corresponding precision. Possible deviations from planarity in the data storage medium and variations in the optical thickness of the protective film covering the burn-film, entail that the distance in practice must be servo-controlled during the fast relative movement between storage medium and lens.
Furthermore, the position of the burn-film is critical. When storing data on rotating discs, the burn-spots are typically positioned in a spiral pattern or in concentrical tracks, or in straight stripes when storing on cards. In both cases, the burn-spots must be placed as close to one another as possible in order that a high storage density be achieved, a typical distance between spots being from a few .mu.m down to approximately 1 .mu.m. The requirements for the positioning of the lens become correspondingly stringent and imply in practice that the data storage medium beforehand has received optical "guide-tracks" which can be followed during recording and reading by a control servo.
During the reading of data, corresponding fundamental limitations apply as during recording: In order to be able to detect the small burn-spots and to discern them from one another, the use of either a laser beam with focusing properties and positioning as mentioned above for recording is required, or an imaging system. In the latter case the resolving power requirements imply that the lens must be positioned with a distance precision corresponding to the one which applies when a laser beam is used.
To summarize: Recording and reading systems which operate with burn-spot diameters of approximately 1 .mu.m or less require in practice a laser as a light source, optical components of high quality, sophisticated mechanical control-systems as well as data carriers with well controlled mechanical and optical properties (cf. below). With regard to commercial competition, this probably has little consequence for larger stationary units in controlled environments, but it may represent a considerable handicap for small, possibly mobile recording and reading units, particularly in difficult environments (vibrations, dust, etc).
The data storage medium itself is also encumbered with limitations and problems: A potential disadvantage of small burn-spot sizes lies in a possible interference of recording and reading due to small dust particles 2 or the like. In principle, this problem can be solved by depositing a transparent protective film 3 onto the burn-film 4, see FIG. 2. Provided that the protective film 3 is thick enough, the laser beam 1 due to its strong focusing, will have such a large diameter on the surface of the protective film 3, that small particles 2 only obscure a small part 6 of the beam 1. This however, requires high quality of the optical properties of the protective film. A general requirement exists for flatness and refractive index constancy in order to avoid focus displacement. This requirement becomes more severe in the case of small burn spots. Thin and flexible data carriers like optical tapes, fiches or cards represent a particularly big problem. The necessary stiffness/flatness can not be built into the data carrier itself, and shadowing effects from (dust) particles cannot be appreciably reduced through focusing as shown in FIG. 2. For recording and reading on/from data storage systems which are based on measuring light reflected from the burn-film, it is usual to apply polarisation sensitive beam-splitters. This means that a protective film in addition to being homogenous, etc., also has to be free from double refraction. This excludes large groups of materials which otherwise would have been eligible e.g. rolled or extruded plastic foils.
Reading by means of reflection from the burn-film requires in addition that it posesses a not insignificant reflection capability. Since the film at the same time must be able to absorb enough radiation during the recording phase, the choice of film material and film thickness is significantly restricted, and a sufficiently good control of reflection/absorption during the production process must be taken care of.
In order to avoid the above mentioned problems and to achieve adaption to special market niches, optical data storage systems have been introduced, based on relatively large burn- spots, i.e. 2.5-25 .mu.m in diameter. In U.S. Pat. No. 4,542,288 and U.S. Pat. No. 4,284,716 Drexler et al. describe an optical data carrier of credit card size, where burn-spots are recorded along a series of straight tracks on a foil with burn-film which is bonded to the card. The burn-spot size is typically from 2.5-7 .mu.m and upwards in diameter, with a distance of 12 .mu.m between the burn tracks. This leads to a significantly lower data storage density than described in the above introduction, and the capacity of the card is thus only 2-4 megabytes. On the other hand, recording and reading become relatively uncritical, and it becomes possible to employ, e.g. an incoherent light source (light emitting diode) for reading. It is intended to introduce optical data cards which are rigid and inexpensive enough to have user characteristics comparable to ordinary credit cards.
A potential problem associated with large burn-spots is the required heating of considerably larger areas by the laser during recording: For example, changing the burn-spot diameter from 0.7 .mu.m to 7 .mu.m leads to an increase of the spot area by a factor of 100. In order to avoid a dramatic and unacceptable increase in the requirements for the laser power or pulse energy, it is therefore necessary to lower the recording threshold of the burn-film (threshold for controlled thermal damage during exposure to laser light) considerably in relation to what is acceptable in stronger focused systems. Drexler et al. have developed a burn-film consisting of silver microparticles in gelatin, which, through chemical treatment, reaches an optimized surface reflection and has a very low recording threshold.
Even though Drexler et al. by use of their special burn-film and the large burn-spot area have achieved technical solutions which are suited for large and important markets, this has resulted in a lowered data storage capacity, which lies approximately two order of magnitude below other optical data storage systems. At present, data storage applications seem to have been targeted that can be met within the 2-4 megabyte capacity of this card limit (e.g. patient journals), but there is no doubt that this low data storage density in the future will be considered as even more limiting and unacceptable, also with regard to small and distributed data systems. Finally, it is until now unclear to which extent the highly sensitive and chemically special burn-film may have poorer stability when exposed to strong influences from adverse environments (heat, light, chemical attacks) than burn-films made from e.g. tellurium.